1. Field of the Invention
The present invention relates to a digital amplifier.
2. Related Art
A digital amplifier is constructed to convert a pulse coded modulation-type (referred to as PCM below) input signal which is obtained by playing a CD or MD into a pulse width modulation-type (referred to as PWM below) input signal, to generate a driving signal by switching an operation voltage by switching means based on such PWM type input signal and to supply an audio signal converted by passing through a low pass filter to audio output means such as a speaker or the like.
Conventionally, a volume adjustment of a digital amplifier is carried out by using a digital attenuator (a digital volume). In other words, before converting the PCM type signal into the PWM type signal, a designated processing (for example, bit shift from the upper digit to the lower digit) is carried out on the PCM type signal (digital data) by the digital attenuator. By carrying out the designated processing on the PCM type signal, if the volume is turned down, more digital data will lack and thus degradation of the output audio quality is unavoidable.
In order to avoid such inconvenience, proposed is a digital amplifier to carry out volume adjustment by controlling the above mentioned operation voltage to be supplied to the above mentioned digital switching means. For example, see Japanese patent application publication JP-A-2001-202696.
As shown in FIG. 8, such digital amplifier includes switching means 300 including a P-channel field effect transistor 302 and an N-channel field effect transistor 304.
Also, the source terminal of the P-channel field effect transistor 302 is connected to a first operation voltage +V, while the source terminal of the n-channel field effect transistor 304 is connected to a second operation voltage −V. The drain terminals of the P-channel field effect transistor 302 and the N-channel field effect transistor 304 are commonly connected to an output terminal. The gate terminals of the P-channel field effect transistor 302 and the N-channel field effect transistor 304 are commonly connected to an input terminal to which the above mentioned PMW type input signal Spwm is inputted.
Then, by turning on and off the field effect transistors 302, 304 in response to the input signal Spmw, the operation voltages +V and −V are switched, thereby outputting a driving signal Sd from the output terminal.
However, in the above mentioned conventional digital amplifier, switching operation is interrupted if the first operation voltage +V is below the minimum gate-source voltage Vgs which is required for switching the P-channel field effect transistor 302.
Therefore, it is possible to normally carry out volume adjustment in the range when the operation voltage exceeds the above mentioned minimum value but volume adjustment is impossible in the range when the operation voltage is below the above mentioned minimum voltage, thereby restricting the range of volume adjustment.
In consideration of the above circumstances, the present invention has been conceived in order to provide an advantageous digital amplifier which is capable of performing volume adjustment over a relatively wide range.